Two gigantic black holes discovered in 'death spiral' collision course

Two gigantic black holes have been discovered on a  collision course that is being described as a "death spiral".

The supermassive black holes each have a mass of more than 800 million times that of our sun and are gradually being drawn together in a "death spiral", according to researchers at Princeton University.

In a paper published on Wednesday (local time) in the Astrophysical Journal Letters, the researchers say the two black holes are sending out large gravitational waves while on their collision course. 

The black holes are roughly 2.5 billion light-years away from Earth, and as looking at distant space objects is like looking back in time - due to the length of time it takes for light to reach Earth - the pair are in a universe 2.5 billion years younger than our own.

The gravitational waves being emitted by the black holes won't reach Earth for billions of years, but the researchers say the black holes are still useful as "their discovery can help scientists estimate how many nearby supermassive black holes are emitting gravitational waves that we could detect right now."

Detecting gravitational waves allows astronomers to gain information on how and even if supermassive black holes merge.

"It's a major embarrassment for astronomy that we don't know if supermassive black holes merge," said co-author Jenny Greene.

When galaxies merge, their respective black holes begin orbiting each other and drawing closer and closer, stealing each other's energy. What happens then is not yet known.

One theory is that when they get to 3.2 light-years apart, they stop moving towards each other, with the slowdown lasting indefinitely - known as the "final parsec problem". In this scenario, only groups of three or more black holes merge.

By studying these new black holes, the scientists hope to understand if this theory is correct.